This invention relates to an improved precipitation process for the production of alumina hydrate from Bayer process sodium aluminate liquors. More particularly, it relates to a precipitation process which produces coarse and strong alumina hydrate at high yields while simultaneously achieving energy savings, as well as reduced equipment and operating costs.
The Bayer process production of alumina hydrate has been practiced since 1888, and the process involves digestion of bauxite with an aqueous caustic medium at elevated temperatures and pressures. Digestion results in a slurry consisting of a liquor containing the alumina values dissolved from the bauxite in the form of sodium aluminate and a caustic-insoluble digestion residue, the so-called "red mud". The red mud is generally separated from the liquor and the alumina content of the liquor is recovered by precipitation. Precipitation is usually induced by seeding the sodium aluminate liquor with solid alumina hydrate and the precipitated alumina hydrate is recovered. Precipitation of alumina hydrate from the sodium aluminate liquor is an involved operation due to the many process variables and the product quality requirements. The process variables involved in the precipitation step, such as temperature, seed charge, holding time, alumina and caustic concentrations, impurity content, etc., affect product quality and yield, and, consequently, this operation requires close control, as well as a thorough understanding of the precipitation process.
The desired characteristics of the alumina hydrate produced by the Bayer process include suitable particle size distribution which is typically measured by screen analysis. To reduce dusting and improve aluminum reduction cell operations, the +325 mesh screen fraction (+44 microns) is used as an industry standard for comparing alumina hydrates for suitability. Generally, only a small quantity of less than 44 micron size particles is allowed. Another factor is strength, which is represented by resistance to abrasion during handling and particularly during calcination, again to avoid excessive dusting. In addition to these quality requirements, it is also important that the alumina hydrate produced by precipitation should be recovered at high yields at a minimum of energy input and at the lowest possible capital equipment cost requirement. Energy savings can be achieved, for example, by conducting the precipitation at relatively low temperatures, i.e., below about 68.degree. C. (155.degree. F.) and equipment costs can be minimized by eliminating the need of a cooling stage between precipitation stages and also by reducing the need for costly precipitators and classifiers which separate the coarse alumina hydrate fraction from the fines.
Over the years, many efforts were made to improve the precipitation stage of the Bayer process to achieve the above-stated goals. In the American Bayer process practice, major emphasis was placed on obtaining a coarse, "sandy" product of high strength. The yield of alumina hydrate, however, was unsatisfactory. In the European Bayer process practice, the yield of alumina hydrate was relatively high in comparison to the American Bayer process; however, the product was too fine and required overcalcination to reduce dustiness. Overcalcination reduces the surface area of the alumina and makes it unsuitable for dry scrubbing in state of the art aluminum reduction facilities. Both processes, the American and the European, had certain advantages, but these advantages were always accompanied by unavoidable difficulties and disadvantages.
During the past few years, several proposals were made to combine the advantages of both of these processes without the accompanying disadvantages. Thus, U.S. Pat. No. 4,234,559 (1980) describes a two-stage precipitation method, each stage proceeding within defined temperature ranges, and to each stage, seed alumina hydrate of different particle size distribution is added to induce precipitation. In the first stage, fine seed is added in such an amount as to provide a defined seed surface area to aluminate liquor ratio, expressed in g/l supersaturation per m.sup.2 seed surface area in the range of 7-25. In the second stage, a larger quantity of coarser seed is added to complete the precipitation. The first precipitation stage is accomplished in a temperature range of 66.degree.-77.degree. C. (151.degree.-171.degree. F.), then the second seeding and precipitation stage is carried out at about 40.degree. C. (104.degree. F.). The process as shown produces a relatively coarse material wherein the fine fraction produced (less than 45 micron size) is less than 15% by weight and under certain process conditions, can be within the range of 4-8% by weight of the produced hydrate. The yield of the alumina hydrate produced according to this patent varies widely depending upon the weight fraction of the fine (less than 45 micron size) alumina hydrate. Thus, a product yield of 71.1 grams Al.sub.2 O.sub.3 /l of liquor is reported at a 14.9% by weight fine fraction, an 83 g/l Al.sub.2 O.sub.3 yield is obtained with an 18.6% by weight fine fraction, and the highest reported Al.sub.2 O.sub.3 yield is 91.7 g/l at a 16.5% by weight fine content. The process disclosed in this patent, although capable of producing higher than conventional yields, is still hampered by the production of an unacceptably high fine fraction as indicated by the examples in the patent. In addition, the temperature drop required between the first and second stages requires extensive interstage cooling, and the use of different size seed materials necessitates the extensive use of expensive classifying equipment.
In U.S. Pat. No. 4,305,913 (1981), a three-stage process is disclosed for the production of coarse and strong alumina hydrate with yields in the range of 70-80 g/l. The process also produces a high percentage of coarse material, at least 90% by weight of the product has a size in excess of 45 microns. The process disclosed in this patent provides a significant advance in the precipitation art, since it is applicable to both the American and European Bayer processes. The need for three distinct but interconnected stages requires significant capital expenditure; in addition, classification associated with each stage adds to processing costs.
In German Offenlegungsschrift No. 3,324,378 (published Jan. 12, 1984), a precipitation process is described wherein coarse alumina hydrate is obtained at reported Al.sub.2 O.sub.3 yields within the range of 77-85 grams/liter. The process shown involves the addition of large seed charges (800-1500 grams seed/liter of sodium aluminate solution) to the supersaturated sodium aluminate solution. The particle size distribution of the product alumina hydrate is controlled by the particle size distribution of the seed charge. Thus, if the fine fraction of the seed charge (less than 44 micron size) constituted 19% by weight of the total charge, the recovered product alumina hydrate also contained 19% by weight of fine fraction. Avoidance of the production of such large quantities of fine alumina hydrate as shown is only achieved by recycling major quantities of the slurry to a classification stage. This process, although capable of increasing Al.sub.2 O.sub.3 yields, does not eliminate the production of large quantities of fines. It also involves the addition of very large seed charges which lead to operational difficulties and requires extensive classification if reduction of the fine content of the product hydrate is desired.
German Patentschrift No. 3,030,631 (first publication Feb. 19, 1981) provides a precipitation process wherein the supersaturated sodium aluminate solution is separated into two streams and each of these streams are separately seeded. Extensive recycling to allow sufficient residence time and, consequently, production of coarse agglomerates, is essential in this process, together with the use of a large number of classifiers to reduce the fine fraction in the product alumina hydrate. While the process allows production of coarse alumina hydrate product, it does so at the cost of yield and it also involves increased operating and capital costs.
It has now been found that alumina hydrate can be produced at high yields in the form of coarse and strong product from supersaturated sodium aluminate liquors resulting from both the American and European Bayer processes. The novel process utilizes a single, relatively small seed charge, which, when added to the entire liquor stream, will allow production of alumina hydrate of desired quality and yield without requiring the extensive recycle systems of the prior art or the need for large-scale use of interstage cooling and classification equipment.